Water resistant corrugated containerboard has long been used to contain perishable or refrigerated products or foods. Where the product has a high moisture content, such as fresh meats or iced seafoods, the water resistance and durability of containerboards in common use is much less than is desired. A box filled with iced fresh fish, for example, is seldom treated with care and if the otherwise water resistant corrugated box is cut or crushed during rough handling, such that the water resistant coating is ruptured, moisture is rapidly wicked into the sidewalls of the container, which then rapidly disintegrates.
Paper manufactured from treated wood fibers is most commonly used for corrugated containerboard and is wax treated to enhance its water resistance when required because the untreated containerboard has little wet strength. A commonly used containerboard comprises a corrugated paper medium spacing and glued to kraft paper liners. These papers are often pretreated with wax or other water resistant agent prior to being formed into the containerboard. The pretreament is not only a costly operation in itself, but the water resistant treatment retards the subsequent gluing of the corrugated medium to the liners during fabrication of the containerboard, as compared to the gluing of untreated liners and medium. For use under high humidity conditions, the fabricated pretreated containerboard is additionally waterproofed, as for example by dipping the corrugated containerboard in a hot melt wax bath or by cascading or curtain coating processes.
In the dipping process, batches of containerboards are lowered vertically into a hot melt bath of wax, then withdrawn into an oven where excess liquid wax drains back into the bath. An air knife may be used to blow excess liquid wax from the surface of each containerboard. Thereafter the wax cools and hardens.
Objections to the dipping process are its slowness, the cumbersome equipment required for handling the containerboards, the difficulty of blowing excess wax uniformly from all of the containerboards in the batch, and more importantly the wasteful and nonuniform distribution of the hardened wax throughout the containerboard. By the nature of the dipping process, the lower ends of the containerboards are first into the bath and last out, with the result of an uneven immersion time and temperature exposure to the hot wax for different parts of the containerboard and a costly uneven distribution of wax, whereby useless wax often clogs the lower portions of the corrugation and piles up in an excessively heavy layer near the lower exterior surfaces, which heavy wax layer is usually a waste and often a hindrance.
I have found that the excess wax does not significantly enhance water resistance and adds weight to the box or container without increasing its strength correspondingly. It is also difficult to glue heavily waxed surfaces together to form a box. Thus where gluing is desired, it is first necessary to scrape or melt the excess wax from the locations to be glued, as described by Lombarde in U.S. Pat. No. 1,536,801. Stapling at such locations in lieu of glue is unsatisfactory because the staples break the water resistant coating and allow water to wick into the containerboard. When the flute openings are clogged with wax, bending of the board at the clogged locations to form a box tears the exterior corrugation liners with consequent impairment of water resistance.
According to the cascade method as described by Stease in U.S. Pat. Nos. 3,635,193 and 3,793,056 and Gjeadel in U.S. Pat. No. 3,343,977, the containerboard is passed vertically in a preheated condition under a cascade of hot liquid wax which runs down the flutes and exterior surfaces of the containerboard. Thereafter the board is cooled to harden the wax. The cascade method relies on gravity flow for the wax which results in uneven exposure of all parts of the containerboard to the wax for equal time intervals and temperature conditions. An uneven distribution of wax over the surfaces of the flutes and the exterior surfaces of the containerboard and a nonuniform impregnation of such surfaces results as the comparatively slow gravity flow of wax congeals on the containerboard. The resulting waxed containerboard is thus subject to most of the objections described in regard to the "dipped" containerboard.
Furthermore, die cutting and scoring of a containerboard transversely of the flutes severely restricts the flute opening and prevents free flow of the wax therealong and is therefore not feasible prior to treatment by either the dipping or the cascade process. Because of the nominal forces available to the dipping and cascade processes for urging the flow of liquid wax longitudinally through the flute openings, these processes cannot avoid heavy accumulations of solid wax in portions of the flute openings, these processes cannot avoid heavy accumulations of solid wax in portions of the flute openings, even when these openings are otherwise unrestricted, and are utterly incapable of achieving satisfactory wax flow through the flute openings when they are restricted by transverse scoring or die cutting. In consequence, the die cutting and scoring required to facilitate formation of a box from the plane containerboard and, which are preferably performed during the same single operation, must be done after the dipping or cascading wax treatment. The scrap from the die cutting, being waxed, cannot be recycled and is thus another source of expensive waste.
The curtain process as described by McConnell et al in U.S. Pat. No. 3,524,759 flows a curtain or cascade of a hot melt water resistant agent on the surface of the containerboard as it passes horizontally under the flow. The curtain process coats only the exterior surfaces of the containerboard, has limited use, and is unsatisfactory for producing containerboard intended for use in humid conditions where there is a likelihood of rupturing the coated surface.
It is well known to the art that overheating of the containerboard during a waterproofing operation will damage the wood fibers, boil out the normal latent water content, which is normally about 6% to 8% but which might range form 2% to 10% of the weight of the untreated containerboard, and render the containerboard too brittle for satisfactory use, such that it cannot be bent as required to form a box without cracking. Accordingly, all attempts to impregnate or coat a corrugated containerboard with a hot melt water repellant, such as melted wax, take care to avoid dessication of the containerboard by prolonged exposure to high temperature. Gonta U.S. Pat. No. 3,692,564 teaches a vertical dipping process using wax at a selected temperature to prevent impregnation by the wax into the interior of the paperboard elements and teaches that such penetration is undesirable and wasteful of wax in column 3. The present invention differs from Gonta '564 by intentionally selecting conditions of wax application which maximize penetration into the interior of the paperboard elements to assure that the interiors are essentially saturated, as explained below.
An important discovery in accordance with the present invention is that once the fibers in the containerboard and the interstices between the fibers are saturated with wax, additional surface layers of wax do not enhance water resistance and from the standpoint of economy of material and efficiency of production are undesirable even though the water resistance remains satisfactory. Such extra, unnecessary wax undesirably increases the weight of the container, and interferes with bending and gluing of parts as desired to fabricate a box.